Handling Input in Java

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system property, they will already have won Then an attacker will nd a way to make a small alteration to a system le or script, or might nd a way to leverage an honest con guration mistake In either case, the lack of input validation becomes a stepping stone the attacker uses on the way to a fullblown system compromise Not all forms of input are equal Input from a con guration le will almost certainly receive different treatment than input from a user But regardless of the source, all input should be subject to validation for at least consistency and syntax Next we walk through examples of security vulnerabilities caused by unvalidated input from sources that are sometimes ignored: configuration files, command-line parameters, database access, and network services We return to the topic of unexpected input sources in other parts of the book 9 discusses cross-site scripting vulnerabilities caused by input from the database, and 12, Privileged Programs, looks at privilegeescalation attacks based on data from temporary files and environment variables Con guration Files Version 1329 of Apache s mod_regex and mod_rewrite modules contain a buffer over ow vulnerability caused by programmers who put too much faith in their con guration [Malo, 2003] A typical Apache con guration allows directory-by-directory con guration through les named htaccess With this setup, users are given the opportunity to add their own configuration files to control the way the contents of each of their directories are displayed The problem with mod_regex and mod_rewrite is that they expected regular expressions in their configuration directives to have nine or fewer capturing groups (Capturing groups are a way of treating multiple characters as a single entity in regular expressions and are indicated by statements enclosed in parentheses) Ten or more capturing groups cause a buffer overflow This is the kind of input the program expects:
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RewriteRule ^/img(*) /var/www/img$1
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But the following input causes a buffer over ow:
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RewriteRule ^/img()()()()()()()()()(*) \ /var/www/img$1$2$3$4$5$6$7$8$9$10
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What to Validate
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Example 51 lists the culprit code The code in bold shows where Apache uses the ten-element array regmatch to hold back references to captures, and where it relies on the unbounded number of captures speci ed in a con guration le, later read into p->regexp->re_nsub, to bound the number of references to write into that xed-size array Example 52 shows how the code was xed by changing both the array and the code that lls it to use the same compile-time constant This bug opens up a number of opportunities for attack First, users who have permission only to upload data to a Web server can now exploit the buffer over ow to run code as the Web server Second, an attacker with no privileges whatsoever now only needs to nd a way to upload a le into the server s content tree to be able to execute code In both cases, a bug in con guration parsing opens the server to new lines of attack
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Example 51 A buffer over ow in Apache A user who can modify an htaccess le can crash the server or execute arbitrary code as the server by writing a regular expression with more than nine capturing groups
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int ap_regexec(const regex_t *preg, const char *string, size_t nmatch, regmatch_t pmatch[], int eflags); typedef struct backrefinfo { char *source; int nsub; regmatch_t regmatch[10]; } backrefinfo; else { /* it is really a regexp pattern, so apply it */ rc = (ap_regexec(p->regexp, input, p->regexp->re_nsub+1, regmatch, 0) == 0);
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Example 52 The x to the Apache buffer over ow The array declaration and the code that lls the buffer now both refer to the same constant
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typedef struct backrefinfo { char *source; int nsub; regmatch_t regmatch[AP_MAX_REG_MATCH]; } backrefinfo; else { /* it is really a regexp pattern, so apply it */ rc = (ap_regexec(p->regexp, input, AP_MAX_REG_MATCH, regmatch,0) == 0);
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